With the advent of the Internet and a decline in computer prices, many people are communicating with one another through computers interconnected by networks. A number of different communication mediums have been developed to facilitate such communications between computer users. One type of prolific communication medium is electronic mail (e-mail).
Unfortunately, because the costs of sending e-mail are relatively low, e-mail recipients are being subjected to mass, unsolicited, commercial e-mailings (colloquially known as e-mail spam or spam e-mails). These are akin to junk mail sent through the postal service. However, because spam e-mail requires neither paper nor postage, the costs incurred by the sender of spam e-mail are quite low when compared to the costs incurred by conventional junk mail senders. Due to this and other factors, e-mail users now receive a significant amount of spam e-mail on a daily basis.
Spam e-mail impacts both e-mail users and e-mail providers. For e-mail users, spam e-mail can be disruptive, annoying, and time consuming. For an e-mail service provider, spam e-mail represents tangible costs in terms of storage and bandwidth usage. These costs may be substantial when large numbers of spam e-mails are sent.
Thus, particularly for large email service providers (ESPs), such as Internet service providers (ISPs) and corporations, it is beneficial to stop spam before it enters the e-mail system. Stopping unwanted e-mails before they enter the system keeps down an ESP's storage and bandwidth costs and provides a better quality of service to the ESP's users. On the other hand, preventing the delivery of wanted e-mail decreases the quality of service to the ESP's users, perhaps to an unacceptable degree, at least from the perspective of the users.
Unfortunately, effective filtering of spam has proved to be difficult, particularly for large ESPs. One reason for the difficulty is the subjective nature of spam, i.e. the decision as to what constitutes span is very subjective in nature. While some categories of unsolicited e-mail, such as pornographic material, are likely to be unwanted and even offensive to the vast majority of people, this is not necessarily true about other categories of unsolicited e-mail. For example, some users may deem all unsolicited invitations to be spam, while other users may welcome invitations to professional conferences, even if such invitations were not explicitly solicited.
Another reason for the difficulty is that there may be some solicited (i.e., wanted) e-mails that closely resemble spam. For example, some e-commerce related e-mails, such as order confirmations, may resemble spam. Likewise, some promotional offers actually may be solicited by the user, i.e. the user may sign-up for promotional offers from a particular merchant.